The invention relates to a paramagnetic measuring instrument comprising a magnetic circuit, formed from ferromagnetic material and having pole ends defining a discontinuity, and a gas measuring chamber disposed in the discontinuity between the pole ends.
More particularly, the present invention relates to a gas measuring chamber for a paramagnetic measuring instrument which includes a frame shaped outer part formed from non-magnetic material and having a central opening extending between the pole ends of the magnetic circuit. Two ferromagnetic pole shoes, each respectively disposed adjacent a separate one of the pole ends and extending into the opening of the frame shaped outer part, have facing surfaces which form plane, parallel pole faces defining, between them, a "measuring gap". Two distribution channels are provided for supplying a measuring gas and a comparison gas, respectively, to the measuring gap. These channels are disposed in the frame shaped outer part on opposite sides of the measuring gap. Each distribution channel has an opening into the measuring gap. A first inlet gas line for supplying the measuring gas and a second inlet gas line for supplying the comparison gas are respectively connected to separate ones of the distribution channels. An outlet gas line is connected to the measuring gap for removing the mixture formed from the measuring and comparison gases.
The function of a paramagnetic measuring instrument of the type described above is based on the paramagnetic properties of certain gases, especially of oxygen. The fundamental principle, on which the measurement is based, as well as arrangements for its application are described in the paper "A New Magnetic Oxygen Meter with a Very Short Response Time and a High Selectivity" by Dr. Hummel in the journal "Chemie - Ingenieur - Technik", 1968, volume 19, pages 947 to 951. In principle, it is a question of alternately supplying the gas to be analyzed and a comparison gas of known composition to the gap of a magnetic circuit and of either determining the periodic changes in magnetic flux on the basis of a mechanically stimulated oscillation of the gases (alternating flow method) or of changing the magnetic flux periodically by electrical means and of measuring this effect on the pressure difference between the two sides of the measuring gap (alternating pressure method). The reference cited is concerned with the last-mentioned, alternating pressure method, whereas the alternating flow method is described in the German Pat. No. 1,079,859.
Methods and arrangements, tested for their practical value, are described in the cited German Pat. No. 1,079,859, as well as in the German Pat. No. 1,648,924. The two principles, on which the measurements are based, as well as a further, improved method of this type are also described in the European published patent application No. 0,177,629. The references cited also show the different possibilities of evaluating the test signals obtained electrically, namely the measurement of the change on the magnetic flux in the alternating flow method by a sensor coil in the magnetic circuit and the change in the pressure difference in the alternating pressure method by a pressure sensor, which is constructed, for example, as a capacitor microphone.
In all cases, there is considerable involvement of gas flows, in which gas oscillations, which are either the cause or the consequence of the effect measured depending on the principle employed for the measurement, are superimposed on the continuous gas flows (for the supply or exchange of gas). Moreover, a volume elements of the gaseous media may have very high flow velocities at least for short periods. As a rule, the measurement and comparison gases are supplied on opposite sides of the measuring gap and the gas mixture of measurement and comparison gases, which is unavoidably, obtained must be removed again continuously from the measuring system. The flow channels, required for this, are furthermore connected with both sides of a membrane or a piston, which either produces the alternating flow or measures the alternating pressure produced.
The measuring chamber of the paramagnetic measuring instrument, which is described generally above, must fulfill three essential requirements.
1. The use of the paramagnetic measurement principle must be possible, that is; there must be as narrow and as precise a measurement gap in the edge region as possible, with a strong field gradient. PA1 2. With respect to the flow prerequisites, the measurement chamber must be so designed that it allows as rapid as possible a continuous gas replacement, as well as a pressure propagation. PA1 3. It must be possible to produce the measuring chamber inexpensively and with great precision and high reproducibility.
The German Pat. No. 3,400,140 already describes an analyzer with a measuring chamber of the type described aboved, which largely fulfills the above requirements. This is made possible by a measuring gap in the form of an extremely flat, hollow cyinder which is surrounded on diametrically opposite sides by sickle-shaped bulges, which serve for supplying and distributing the measurement and comparison gases as well as for making the connection with the pressure measuring device (capacitor microphone). This shape is achieved owing to the fact that circular cylindrical pole shoes, which, between themselves, enclose the cylindrical measuring gap, are inserted from opposite sides into an oval recess in an external part of the measuring chamber. On the one hand, in this connection, it is desirable that the sickle-shaped bulges or distribution chambers be led as far as possible around the measuring gap in order to be able to have the gases flow through as large a portion of the inhomogeneous magnetic filed (field gradient) in the edge region of the measuring gap as possible. On the other hand, the mixing of the measurement and comparison gases in the ends of the sickle-shaped bulges, before they enter the measuring gap, must be prevented. In the sickle-shaped distribution chambers of the German Pat. No. 3,400,140, flow paths of greatly different lengths and different distances naturally arise between volume elements of measuring gas on the one hand and comparison gas on the other, lying opposite to one another in the direction of flow.
It is therefore an object of the invention to further improve the analyzer or measuring chamber of German Pat. No. 3,400,140 with regard to the flow relationships and the manufacturing possibilities.